Difficulty: Introductory | Prerequisites: Mendelian Genetics Foundations notes (Part 1)
Once you know what genes, alleles, and dominance are (Part 1), the next step is learning to read and write the notation. This unit focuses on connecting an organism's genetic makeup (genotype) to what you actually observe (phenotype), and on practising that skill with human traits. Being fluent in genotype notation is essential before you move on to Punnett squares and probability calculations.
Genotypes are written as two letters (one per allele): capitals for dominant, lowercase for recessive. The genotype determines the phenotype. If you can see a recessive trait, the genotype must be homozygous recessive; if you can see the dominant trait, the genotype is either homozygous dominant or heterozygous, and you may need more information (a cross, or family history) to tell which.
Homozygous dominant
A genotype with two copies of the dominant allele (e.g. BB). The organism expresses the dominant phenotype.
Think of it as a "double dose" of the dominant version.
Homozygous recessive
A genotype with two copies of the recessive allele (e.g. bb). The organism expresses the recessive phenotype.
This is the only genotype that lets the recessive trait show.
Heterozygous
A genotype with one dominant and one recessive allele (e.g. Bb). The organism expresses the dominant phenotype.
In simple terms, the dominant allele "wins" over the recessive one in determining appearance.
Carrier
An organism that is heterozygous for a recessive trait. It shows the dominant phenotype but carries one copy of the recessive allele and can pass it to offspring.
A carrier looks normal for that trait but has a hidden recessive allele tucked away.
A genotype is written as two letters, each representing one allele for the trait.
The dominant allele is always capitalised (e.g. B); the recessive allele is always lowercase (e.g. b).
Three possible combinations exist for any trait: homozygous dominant (BB), heterozygous (Bb), and homozygous recessive (bb).
When writing a heterozygous genotype, the capital letter is written first by convention (Bb, not bB).
BB or Bb: the dominant phenotype is expressed. You cannot tell these apart just by looking.
bb: the recessive phenotype is expressed. This is the only genotype that produces the recessive trait.
If a question gives you a recessive phenotype, the genotype is always homozygous recessive. If it gives you a dominant phenotype, you know at least one allele is dominant, but you may need a cross or pedigree to determine whether the organism is BB or Bb.
The traits below are treated as simple dominant/recessive in this course. In reality, most involve polygenic or more complex inheritance, but for exam purposes, use this simplified model.
Trait | Dominant phenotype | Recessive phenotype |
|---|---|---|
Hair colour (dark/light) | Dark hair | Light hair |
Hair colour (red) | Red hair | Non-red hair |
Eye colour (brown/blue) | Brown | Blue |
Eye colour (green/blue) | Green | Blue |
Tongue rolling | Roller | Non-roller |
Widow's peak | Peak | No peak |
Ear lobes | Free (detached) | Attached |
Suppose you have light hair. Light hair is the recessive phenotype for the dark/light hair trait. Because recessive traits require two recessive alleles, your genotype must be ll (homozygous recessive).
Suppose instead you have dark hair. Dark hair is dominant, so your genotype is either LL (homozygous dominant) or Ll (heterozygous). Without knowing your parents' genotypes or doing a test cross, you cannot distinguish between the two from phenotype alone.
Students often think that if you show the dominant phenotype, your genotype must be homozygous dominant. It could equally be heterozygous. The only phenotype that tells you the exact genotype is the recessive one.
Students sometimes write heterozygous genotypes with the lowercase letter first (bB). Convention is always capital first (Bb).
Students assume "carrier" means the person is sick or affected. A carrier shows no sign of the recessive trait; they simply carry one copy of the recessive allele.
The human trait table is a simplification. Do not assume these rules apply outside this course without checking.
⚠️ Expect questions that give you a phenotype and ask for the genotype. For recessive phenotypes, the answer is always homozygous recessive. For dominant phenotypes, state both possibilities (homozygous dominant or heterozygous) unless more information is given.
⚠️ Know the human trait table. A question may ask you to determine your own phenotype and possible genotype for several traits.
⚠️ Be precise with notation: capital for dominant, lowercase for recessive, capital letter first in heterozygous genotypes.
Fill in the blank: A genotype written as Bb is called __________. (Heterozygous.)
True or False: If you have attached earlobes, your genotype could be Ee. (False: attached is recessive, so the genotype must be ee.)
Fill in the blank: Dominant alleles are written as __________ letters. (Capital / uppercase.)
True or False: A person who is Bb for a trait is a carrier of the recessive allele. (True.)
Fill in the blank: If a person shows a recessive phenotype, their genotype must be __________ __________. (Homozygous recessive.)
Q: A student has brown eyes. What are their possible genotypes for the brown/blue eye colour trait?
A: BB (homozygous dominant) or Bb (heterozygous). Both produce the dominant brown-eye phenotype.
Q: A student has blue eyes. What is their genotype?
A: bb (homozygous recessive). Blue is the recessive phenotype, so both alleles must be recessive.
Q: A person who can roll their tongue (Tt) has children with a non-roller (tt). What proportion of their children would you expect to be non-rollers?
A: 50%. The cross Tt x tt produces offspring that are half Tt (rollers) and half tt (non-rollers).
Q: Why can you not determine whether a dark-haired person is LL or Ll from phenotype alone?
A: Because both LL and Ll produce the same dominant phenotype (dark hair). The dominant allele masks the recessive one in a heterozygous individual.
Q: A person with no widow's peak (ww) has a child with a person who has a widow's peak. The child has no peak. What is the peaked parent's genotype?
A: Ww (heterozygous). The no-peak child is ww, meaning it received a w from each parent. The peaked parent must have had a w to give, so they cannot be WW.
This material feeds directly into Punnett squares, where you will use genotype notation to predict offspring ratios. It also connects to pedigree analysis, where you work backwards from phenotypes in a family tree to deduce unknown genotypes. Later in the course, incomplete dominance and codominance will show what happens when the simple dominant/recessive model does not fully apply.
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